Theoretical Investigation Of The Thermoelectric Properties Of Several Layered Materials

Thermoelectric effect can be used to achieve the direct conversion of heat and electricity without environmental pollution.The efficiency of thermoelectric materials is the key point,normally char-acterized by the figure of merit ZT=?S²?T?/?,in which S is the Seebeck coefficient,?is the electrical conductivity and?is the thermal conductivity,and T is the absolute temperature.That means a good thermoelectric material should have high Seebeck coefficient,high electrical conductivity and low thermal conductivity.But the three parameters are coupled inherently and it is difficult to opti-mize one without having negative effects on another.In the 1950s,semiconductor materials Bi₂Te₃,PbTe and other narrow bandgap semiconductors were found to have good thermoelectric properties and were called traditional thermoelectric materials,which have been widely studied.In recent years,due to the development of nanotechnology,a series of new thermoelectric materials are discovered.Among them are the“electronic-crystal phonon-glass“materials show low thermal conductivity with electrical transport properties unaffected.On the other hand,Hicks and Dresselhaus proposed that di-mensionality reduction can improve the thermoelectric properties of materials.In the low dimensional materials,the effect of quantum confinement may change the density of state profile near the Fermi surface,which improves the material's power factor,and the finite size effect may enhance phonon scattering and reduce the thermal conductivity.Improving the power factor and reducing the thermal conductivity are the effective approaches to improve the figure of merit.Our work is carried out based on the above considerations.Firstly,we study the thermoelectric property of monolayer GaX?X=S,Se,Te?,and find that a spe-cial Mexican-hat-shaped dispersion appears at the top of the valence band.The special dispersion sharpens the density of states near the Fermi surface,which can increase the Seebeck coefficient effec-tively.We use a simple Hamiltonian to analyze the effect,and find that it can be regarded as a quasi-one-dimensional band structure in two-dimensional materials.Furthermore,with strain engineering,the position of the conduction band bottom can be tuned.That means we can tune the thermoelectric properties with strain engineering.Next,we study the thermoelectric property of layered Bi₂O₂X?X=Se/Te?.Because of the elec-trostatic interaction between sublayers,the vibrations of Bi and Se/Te atoms couple strongly,which can give rise to phonon anharmonic scattering and reduction of the thermal conductivity.The ther-moelectric power factor of p-doped sample is much larger than that of the n-doped case assuming the same relaxation time.Thus,effective p-doping is one possible way to achieve high thermoelectric performance in these materials.Usually,the temperature dependence of lattice constant due to thermal expansion is neglected in the study of thermoelectric properties of materials.And the influence of this situation on the ther-moelectric properties of materials is not clear.In order to study this question,in the last part of this article,we study the thermal properties of two-dimensional honeycomb lattices,including graphene,silicene,germanene,and blue phosphorene.From the lattice structure and phonon spectrum,we study the similarities and differences of their properties.We find that,from graphene to blue phosphorene,a phonon band gap develops due to buckling-induced mixing of the in-plane and out-of-plane phonon modes.At room temperature,they exhibit negative thermal expansion.Because the ZA mode of two-dimensional materials is soft,the calculated thermal expansion coefficient depends sensitively on the stress applied.It is more accurate to use the Grüneisen theory with small applied strain to calculate thermal expansion coefficient.Based on the above results,we will study the influence of thermal ex-pansion on thermoelectric properties in the future.